Showing papers on "Granulite published in 1993"

Exhumation of high-pressure rocks: a review of concepts and processes

Abstract: The exhumation of high-pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures, or the transport of the metamorphic rocks through the overburden Exhumation cannot be achieved simply by thrusting or strike-slip faulting It may be caused by erosion of shortened and thickened crust, but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km One or more of the following additional mechanisms may be involved 1 Corner flow of low-viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock, and may explain some occurrences of high-pressure tectonic blocks in melange This process does not, however, appear to be adequate to explain the exhumation of regional high-pressure terrains 2 Buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings This is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle, but cannot explain the emplacement of coherent tracts of high-density metamorphic rock into shallow crustal levels Some high-pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low-density mud or serpentinite 3 Extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high-pressure terrains Extension may occur in the upper part of an accretionary wedge thickened by underplating; or it may affect the whole lithosphere in a region of intracontinental convergence, if surface elevation has been increased by the removal of a lithospheric root In the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain

Tectonics of an ultrahigh-pressure metamorphic terrane: The Dabie Shan/Tongbai Shan Orogen, China

Abstract: Ultrahigh-pressure metamorphic rocks with coesite and diamond form a tectonic slice over 20 km thick, called the eclogite zone, within the Dabie Shan complex in the Qinling orogen in central China. The orogen separates the Sino-Korean block in the north from the Yangtze block in the south. The Dabie Shan Complex is a composite terrane made up of eclogite facies and amphibolite facies gneiss slices and represents fragments of the lower continental crust of the Yangtze block. The Dabie Shan Complex is bounded in the south by a Triassic foreland fold-thrust belt and in the north by a greenschist facies metaclastic unit, the Foziling Group, which probably represents the passive continental apron deposits of the Yangtze block. Farther north is a granulite facies gneiss complex, the Qinling Group, which has ultramafic slivers and includes the remnants of an island arc with two bounding suture zones. North of the Qinling Group are early Paleozoic active margin deposits of the Sino-Korean block. The eclogite zone in the Dabie Shan Complex is sandwiched between amphibolite facies gneiss slices. Dating by Sm-Nd, Rb-Sr, and Ar-Ar of two eclogite samples from the eclogite zone gives early to middle Triassic ages (236–246 Ma); the initial eNd values indicate reworking of a 2.11 and 1.55 Ga continental crust. A Himalayan-type tectonic evolution is envisaged for the Qinling orogen with the creation of a 100-km-thick crustal thrust wedge through continuous underplating during the subduction of the Yangtze continental lithosphere. Exhumation of the ultrahigh-pressure metamorphic rocks was chiefly achieved by the southward propagation of the thrust planes, thereby isostatically uplifting and eroding the earlier deeply subducted parts of the orogen. A total of 680 km of southward thrusting in front of Dabie Shan is inferred, based on the abrupt termination of the Tanlu fault. Normal faulting possibly caused by gravitational collapse probably also had a role in the exhumation process.

Age of younger tonalitic magmatism and granulitic metamorphism in the South Indian transition zone (Krishnagiri area); comparison with older Peninsular gneisses from the Gorur–Hassan area

Abstract: A major episode of continental crust formation, associated with granulite facies metamorphism, occurred at 2.55–2.51 Ga and was related to accretional processes of juvenile crust. Dating of tonalitic–trondhjemitic, granitic gneisses and charnockites from the Krishnagiri area of South India indicates that magmatic protoliths are 2550–2530 ± 5 Ma, as shown by both U–Pb and 207Pb/206Pb single zircon methods. Monazite ages indicate high temperatures of cooling corresponding to conditions close to granulite facies metamorphism at 2510 ± 10 Ma. These data provide precise time constraints and Sr–Nd isotopes confirm the existence of late tonalitic–granodioritic juvenile gneisses at 2550 Ma. Pb single zircon ages from the older Peninsular gneisses (Gorur–Hassan area) are in agreement with some previous Sr ages and range between 3200 ± 20 and 3328 ± 10 Ma. These gneisses were derived from a 3.3–3.5-Ga mantle source as indicated from Nd isotopes. They did not participate significantly in the genesis of the 2.55-Ga juvenile magmas. All these data, together with previous work, suggest that the 2.51-Ga granulite facies metamorphism occurred near the contact of the ancient Peninsular gneisses and the 2.55–2.52-Ga ‘juvenile’tonalitic–trondhjemitic terranes during synaccretional processes (subduction, mantle plume?). Rb–Sr biotite ages between 2060 and 2340 Ma indicate late cooling probably related to the dextral major east–west shearing which displaced the 2.5-Ga juvenile terranes toward the west.

Granulites in the Tongbai Area, Qinling Belt, China: Geochemistry, petrology, single zircon geochronology, and implications for the tectonic evolution of eastern Asia

Abstract: The Tongbai area of the eastern Qinling belt in China includes granulite-grade metamorphic assemblages (Qinling Complex) which were previously regarded as Archean to early Proterozoic in age and belonging to the southern margin of the North China plate (craton) Our petrological and geochemical data characterize these rocks as two-pyroxene granulites and garnet granulites which formed at temperatures of 757°–840°C and pressures of about 95 kbar and are now found as xenoliths in granodioritic gneisses The protoliths of these rocks were granodiorites and tholeiitic basalt or gabbro The 207Pb/206Pb ratios derived from evaporation of single zircons yield ages of 470±20 and 470±14 Ma, respectively, for the basic granulites which we interpret to reflect the time of protolith emplacement These are intruded by a 435±14 Ma granodioritic gneiss post-dating granulite formation A metaquartzite sample contains detrital zircons as old as 2555±8 Ma Two samples of granitoid gneiss from the Tongbai Complex S of the Qinling granulites have single-zircon 207Pb/206Pb evaporation ages of 776±8 and 746±10 Ma, respectively, and document late Proterozoic igneous activity We suggest that the Qinling granulites document an important and hitherto unknown phase of early Silurian crustal thickening following subduction and continental collision and that both the Qinling and Tongbai Complexes were part of the southern margin of the North China craton prior to this event and record late Proterozoic igneous activity

P–T–t evolution of orogenic belts and the causes of regional metamorphism

Abstract: Barrow (1893) introduced three important ideas that furthered understanding of metamorphic processes: (i) the use of critical index minerals in argillaceous rocks to define metamorphic zones and elucidate spatial features of regional metamorphism; (ii) the concept of progressive metamorphism; and (iii) the concept of magmatic advection of heat as a possible cause of regional metamorphism. This article expands upon these themes by reviewing our understanding of the dynamic evolution of orogenic belts as interpreted from the P–T–t paths of metamorphic rocks, and by considering the likely causes of the different kinds of regional metamorphism that we observe within orogenic belts. Understanding metamorphic rocks allows the distinction of two fundamentally different types of orogenic belt defined by relative timing of maximum T and maximum P. Orogenic belts characterized by clockwise P–T paths achieved maximum P before maximum T, the metamorphic peak normally post-dated early deformation within the belt and additional heating above the ‘normal’ conductive flux has been related to the amount of overthickening. By contrast, orogenic belts characterized by counterclockwise P–T paths achieved maximum T before maximum P, the metamorphic peak normally pre-dated or was synchronous with early deformation within the belt and additional heating above the ‘normal’ conductive flux has been related to the emplacement of plutons. Techniques used to constrain portions of P–T–t paths include: the use of mineral inclusion suites in porphyroblasts and reaction textures; thermobarometry; the use of fluid inclusions; thermodynamic approaches such as the Gibbs method; radiogenic isotope dating; fission track studies; and numerical modelling. We can utilize specific mineral parageneses in suitable rocks to determine individual P–T–t paths, and a set of P–T–t paths from one orogenic belt allows us to interpret the spatial variation in dynamic evolution of the metamorphism. Recent advances are reviewed with reference to collision metamorphism, high-temperature–low-pressure metamorphism, granulite metamorphism, and subduction zone metamorphism, and some important directions for future work are indicated.

U-Pb geochronology of the Grenville Orogen of Ontario and New York: constraints on ancient crustal tectonics

Abstract: Based on lithological, structural and geophysi- cal characteristics, the Proterozoic Grenville Orogen of southern Ontario and New York has been divided into domains that are separated from each other by ductile shear zones In order to constrain the timing of meta- morphism, U-Pb ages were determined on metamorphic and igneous sphenes from marbles, calc-silicate gneisses, amphibolites, granitoids, skarns and pegmatites In addi- tion, U-Pb ages were obtained for monazites from meta- pelites and for a rutile from an amphibolite These mineral ages constrain the timing of mineral growth, the duration of metamorphism and the cooling history of the different domains that make up the southern part of the exposed Grenville Orogen Based on the ages from metamorphic minerals, regional and contact metamorphism occurred in the following intervals: Central Granulite Terrane:

Coesite-bearing granulite retrograded from eclogite in Weihai, eastern China

Abstract: Coesite in a granulite host-rock is reported from Weihai City, northeastern Shandong Province, China. Coesite occurs as inclusions in garnet, which is separated from the fine-grained matrix of quartz + clinopyroxene + homblende by a zoned corona of plagioclase + clinopyroxene + orthopyroxene + hornblende. Petrological evidence shows that the coesite-bearing granulite is retrograded from eclogite. Three stages in the metamorphic history of the granulite have been recognized. (i) Coesite-eclogite stage: The assemblage reconstructed from the inclusions in garnet is Mg-rich garnet + omphacite + coesite + rutile. (ii) During the granulite stage, these minerals reacted to form quartz + clinopyroxene + hornblende (in the matrix), clinopyroxene + albite (omphacite pseudomorphs) and plagioclase + clinopyroxene + orthopyroxene + hornblende (as corona around garnet). (iii) Amphibolite stage: Earlier minerals were replaced by biotite, epidote and hornblende. The P-T conditions estimated with various geothermobarometers show a T-increasing and P-decreasing path from the eclogite stage (T ∼ 720°C, P > 28 kbar) to the granulite stage (T ∼ 850°C, P ∼ 10 kbar). Thus the exhumation of the ultra-high-pressure rocks in Weihai proceeded under increasing temperature, and may not have been as fast as it was thought before

The effect of fluid and deformation on zoning and inclusion patterns in poly-metamorphic garnets

Abstract: Within the Bergen Arcs of W Norway, Caledonian eclogite facies assemblages (T≥650°C, P≥15 kbar) have formed from Grenvillian granulites (T= 800–900°C, P≥10 kbar) along shear zones and fluid pathways. Garnets in the granulites (grtI: Pyr56–40 Alm45–25Gro19–14) are unzoned or display a weak (ca. 1 wt% FeO over 1000μm) zoning. The eclogite facies rocks contain garnets inherited from their granulite facies protoliths. These relict garnets have certain areas with compositions identical to the garnets in their nearby granulite, but can be crosscut by bands of a more Almrich composition (grtII: Pyr31–41Alm40–47Gro17–21) formed during the eclogite facies event. These bands, orientated preferentially parallel or perpendicular to the eclogite foliation, may contain mineral filled veins or trails of eclogite-facies minerals (omphacite, amphibole, white mica, kyanite, quartz and dolomite). Steep compositional gradients (up to 9 wt% FeO over 40 μm) separate the two generations of garnets, indicating limited volume diffusion. The bands are interpreted as fluid rich channels where element mobility must have been infinitely greater than it was for the temperature controlled volume diffusion at mineral interfaces in the granulites. The re-equilibration of granulite facies garnets during the eclogite facies event must, therefore, be a function of fracture density (deformation) and fluid availability. The results cast doubts on modern petrological and geochronological methods that assume pure temperature controlled chemical re-equilibration of garnets.

Origin and significance of tourmaline-rich rocks in the Broken Hill district, Australia

Abstract: Tourmaline-rich rocks are widespread minor lithologies within the Early Proterozoic Willyama Supergroup in the Broken Hill district, Australia. Tourmaline concentrations occur in strata-bound and local stratiform tourmalinites, clastic metasedimentary rocks, quartz-gahnite lode rocks, stratiform Pb-Zn-Ag sulfide ores, garnet quartzites, strata-bound scheelite deposits, quartz-tourmaline nodules, discordant quartz veins, and granitic pegmatites. Most of the tourmaline-rich rocks are within the Broken Hill Group that hosts the main Pb-Zn-Ag ores.At the Globe mine along the northeast end of the main lodes, tourmalinites are closely associated with Pb-Zn-Ag mineralization and in places are interbedded with Mn-rich garnet quartzites. Galena and other ore minerals occur locally in the cores of recrystallized tourmaline grains in these tourmalinites, indicating that tourmaline and sulfides were present together prior to deformation and high-grade metamorphism. Electron microprobe analyses of tourmalines intergrown with Fe sulfides at the Globe mine show Mg-rich compositions relative to tourmalines in sulfide-free assemblages from the same area, suggesting early (premetamorphic) introduction of boron and Mg enrichment of tourmaline by sulfide-silicate reactions during metamorphism.Combined field and geochemical data indicate that the district tourmalinites represent normal clastic sediments that were metasomatically altered by boron-rich hydrothermal fluids at or below the sediment-water interface. Whole-rock chemical analyses of 33 tourmaline-rich rocks show linear trends of data for major and trace elements that closely resemble the trends observed for unmineralized elastic metasedimentary rocks of the district. Average Fe/Al, Mg/Al, Na/Al, and Ti/Al molar ratios of the tourmaline-rich rocks and clastic metasediments are very similar; the average K/Al molar ratio of the tourmaline-rich rocks is significantly lower than that of the clastic metasediments, reflecting the loss of K during tourmalinite formation. Chondrite-normalized patterns of rare earth elements (REE) in the quartz-rich tourmalinites are generally similar to those of the clastic metasediments, except for minor depletions of light REE; local positive and negative Ce anomalies suggest tourmalinite formation in the presence of seawater or a seawater-derived pore fluid. The geochemical data imply relative immobility of Al, Ti, Cr, and heavy REE during hydrothermal alteration and later metamorphism. Boron isotope analyses of 52 tourmaline separates show a total range of delta 11 B values from -26.8 to -17.0 per mil. Fine-grained, euhedral, nonpoikilitic tourmalines from tourmalinites in the andalusite-muscovite zone in the northern part of the district (e.g., Black Prince mine) have delta 11 B values from -21 to -17 per mil, whereas coarse granoblastic and poikilo-blastic tourmalines from the sillimanite and two-pyroxene granulite zones in the southern part of the district (e.g., Globe mine) have delta 11 B values of-24 to -19 per mil. Tourmalines in strongly retrogressed tourmalinites have delta 11 B values from about -27 to -20 per mil. The observed variations in delta 11 B are consistent with prograde and retrograde metamorphic fractionation of boron isotopes, in which the fluid phase is preferentially enriched in the heavier isotope ( 11 B). Premetamorphic hydrothermal fluids that deposited the Black Prince tourmalinites had delta 11 B values of-8 to -5 per mil at 200 degrees to 300 degrees C, suggesting a boron source from nonmarine evaporite borates.Tourmalinites in the Broken Hill district apparently formed by the same submarine hydrothermal processes as the main Pb-Zn-Ag lodes and the siliceous ferromanganese protoliths of the garnet quartzites. In our model, the hydrothermal system(s) acquired abundant boron by leaching evaporitic borates within the Thackaringa Group, the stratigraphic sequence that underlies the Broken Hill Group and most of the tourmaline concentrations. We suggest that evaporites of the Thackaringa Group provided a source of readily extractable boron for formation of the tourmalinites and also the source of the fluoride, sulfur, and perhaps the carbonate in the main lodes; such evaporites may have been critical for increased metal chloride complexing and transport necessary for deposition of the high-grade Pb-Zn-Ag ores. The Broken Hill deposit may have formed contemporaneously with the Mount Isa and McArthur River Pb-Zn-Ag deposits in similar evaporite-bearing sequences during widespread Early Proterozoic continental rifting.

Zircon ages and the distribution of Archaean and Proterozoic rocks in the Rauer Islands

Abstract: The application of zircon U-Pb geochronology using the SHRIMP ion microprobe to the Precambrian high-grade metamorphic rocks of the Rauer Islands on the Prydz Bay coast of East Antarctica, has resulted in major revisions to the interpreted geological history. Large tracts of granitic orthogneisses, previously considered to be mostly Proterozoic in age, are shown here to be Archaean, with crystallization ages of 3270 Ma and 2800 Ma. These rocks and associated granulite-facies mafic rocks and paragneisses account for up to 50% of exposures in the Rauer Islands. Unlike the 2500 Ma rocks in the nearby Vestfold Hills which were cratonized soon after formation, the Rauer Islands rocks were reworked at about 1000 Ma under granulite to amphibolite facies conditions, and mixed with newly generated felsic crust. Dating of components of this felsic intrusive suite indicates that this Proterozoic reworking was accomplished in about 30–40 million years. Low-grade retrogression at 500 Ma was accompanied by brittle shearing, pegmatite injection, partial resetting of U-Pb geochronometers and growth of new zircons. Minor underformed lamprophyre dykes intruded Hop and nearby islands later in the Phanerozoic. Thus, the geology of the Rauer Islands reflects reworking and juxtaposition of unrelated rocks in a Proterozoic orogenic belt, and illustrates the important influence of relatively low-grade fluid-rock interaction on zircon U-Pb systematics in high-grade terranes.

Experimental high-pressure granulites: Some applications to natural mafic xenolith suites and Archean granulite terranes

Abstract: High-pressure granulite assemblages have been produced as residues in partial melting experiments on a natural (alkalic basalt) amphibolite between pressures of 12 and 18 kbar. In particular, at 18 kbar, partial melting of the hornblende + plagioclase ± quartz assemblage under fluid-absent conditions produces garnet + clinopyroxene + new albitic plagioclase + melt. Seismic velocities ( V P ) are estimated from the modal data for the experimental assemblages and range from 6.90 km/s for hornblende-bearing residues to 7.62 for the dominantly garnet- clinopyroxene residues. These values are typical for rock types in the lowermost crust, transitional to mantle. The experimental results help place additional pressure-temperature- a (H 2 O) constraints on the source region for the natural high-pressure granulites. The experimental residue assemblage formed at 18 kbar has been described in natural xenolith suites from the Delegate Pipes in Australia, where the pipes intrude Upper Ordovician and Lower Devonian continental crust, and in the Bearpaw Mountains, Montana, where the xenolith- bearing magmas intrude older crust of Archean age. The combination of these data shows that the xenoliths may indeed represent lowermost continental crust and furthermore helps interpret the nature of the crust-mantle boundary in these areas. The Delegate Pipes xenoliths suggest that the crust-mantle boundary may be the site for partial melting and assimilation, whereas the Bearpaw Mountains samples indicate that magmatic underplating may have been a major process in generating thick continental Archean crust. The experimental data and Bearpaw Mountains xenoliths suggest that the large range of rock types found in Archean granulite terranes may not be representative of the lowermost continental crust.

Thermobarometric constraints on the depth of exposure and conditions of plutonism and metamorphism at deep levels of the Sierra Nevada Batholith, Tehachapi Mountains, California

Abstract: We present thermodynamic estimates of pressures, temperatures, and volatile activities in variably deformed, gabbroic to granitic, Cretaceous (115–100 Ma) batholithic and framework rocks of the Tehachapi Mountains, southernmost Sierra Nevada, California. Al contents of hornblende in granitoids imply igneous emplacement at ∼8 kbar in the southernmost Tehachapi Mountains, with lower pressures (3–7 kbar) to the north. Metamorphic pressures and temperatures for garnet-bearing paragneisses and metaigneous rocks were estimated on the basis of garnet-hornblende-plagioclase-quartz and garnet-biotite-plagioclase-quartz thermobarometers. Disparate results for the metaigneous rocks from the latter system point to the difficulty of applying pelite-based thermobarometers to rocks of contrasting composition and mineralogy. Preferred pressures cluster at 7.1–9.4 and 3.6–4.3 kbar. Incomplete knowledge of reaction histories, however, limits our interpretation of the lower pressures because they are minimum estimates. The ∼4-kbar samples are all from a small area and, if our interpretation is correct, they imply a local, more shallow event superimposed on crust once residing at deeper structural levels. Garnet-hornblende and garnet-biotite temperatures are less coherent, likely owing to retrograde Fe-Mg exchange, and range from 570° to 790°C, The majority of the rocks are igneous and affected by recrystallization and metamorphism during subsolidus cooling; they are not granulites. Country rock paragneisses are typically migmatized at “peak” metamorphic conditions near that of the wet granite solidus (>690°C). Veinlike paragenesis of garnet in the metaigneous rocks suggests formation related to the presence of a fluid phase. Thermodynamic estimates of volatile activities in these garnet-bearing assemblages suggest variable, mostly CO_2-rich fluid compositions, in the absence of any pervasive fluid flux. The igneous rocks of the Tehachapi Mountains were thus intruded at depths of ∼30 km, making them the deepest known exposed components of the Cretaceous Sierra Nevada batholith. Metamorphism occurred at these great depths and, perhaps, locally after ∼15 km of uplift before ∼87 Ma, implying an uplift rate of 1.2 mm/yr. (A minimum uplift rate is 0.6 mm/yr.) This original uplift and possible subsequent uplift events may have been related to underthrusting of a block of Rand Schist from what is now the southeast, with concomitant widespread ductile deformation. The deduced pressure-temperature and uplift history is similar to those of high-pressure/high-temperature Cretaceous batholithic rocks in Salinia and the San Gabriel Mountains, but direct correlation is not wan-anted. When compared with higher-level intrusive rocks from analogous portions of the Sierra Nevada batholith to the north, the Tehachapi rocks reveal a deep batholith that is more heterogeneous and somewhat more mafic on average, but displaying a similar level of isotopic hybridization involving mantle and crustal sources. The batholith is quartz-rich at these levels, suggestive of a weak, ductile middle crust susceptible to prolonged deformation and possible delamination.

Tectonometamorphic evolution of the Bohemian Massif: evidence from high pressure metamorphic rocks

Abstract: The Variscan orogenic belt, of which the Bohemian Massif is a part, is typically recognized for its characteristic low pressure, high temperature metamorphism and a large volume of granites. However, there are also bodies of high pressure rocks (eclogites, garnet peridotites and high pressure granulites) which are small in size but widely distributed throughtout the Massif. Initially the high pressure rocks were considered to be relicts of a much older orogenic event, but the increasing data derived from isotopic and geochronological investigations show that many of these rocks have Palaeozoic protoliths. Metamorphic ages from the high pressure rocks define no single event. Instead, a number of discrete clusters of ages are found between about 430 Ma and the time of the dominant low pressure event at around 320–330 Ma.

Age of Metamorphism in the High-Grade Rocks of Sri Lanka

Abstract: Controversy concerning the age of regional granulite-grade metamorphism in Sri Lanka stems from the fact that different isotopic systems apparently record different events. Published isotopic data show that zircons from igneous and sedimentary protoliths experienced considerable Pb-loss at 550 Ma and that new growth of zircon, monazite, rutile and garnet between $$539 \pm 6 and 608 \pm 3 Ma$$ probably dates near-peak metamorphic conditions. Upper concordia intercept ages between ~670 and ~1950 Ma for magmatic zircons from high-grade orthogneisses and charnockites date igneous emplacement and also support the occurrence of granulite metamorphism after ~670 Ma. A granulite of igneous origin from the Kataragama klippe in southern Sri Lanka yielded discordant ion microprobe data that suggest emplacement of the protolith at 1770-1880 Ma. Thus a Rb-Sr whole-rock age of $$1930 \pm 130 Ma$$ from the same suite is unlikely to reflect isotopic homogenization during granulite formation, and we interpret this and oth...

P–T–t paths and tectonic history of an early Precambrian granulite facies terrane, Jining district, south‐east Inner Mongolia, China

Abstract: The widespread khondalite series of south-east Inner Mongolia consists largely of biotite–sillimanite–garnet gneiss and quartzo-feldspathic gneiss with some marble and mafic granulite layers. It has experienced two metamorphic events at c. 2500 and 1900–2000 Ma. A pre-peak stage of the first metamorphism at T= 600–700°C and P > 6–7 kbar is recognized by the relict amphibolite facies assemblage Ky–Grt–Bt–Pl–Qtz and ‘protected’inclusions of biotite, hornblende, sodic plagioclase and quartz in garnet or orthopyroxene. The peak stage, with T=c. 800 ± 50°C and P 8–10 kbar, is characterized by the widespread granulite facies assemblages Sil–Grt–Bt–Kfs–Pl–Qtz in gneiss and Opx–Cpx–Pl ± Hbl ± Grt in granulite. The P–T–t path suggests that the supracrustal sequence was buried in the lower crust by tectonic thickening during D1–D2. The beginning of the second metamorphism is characterized by further temperature rise to 700°C or more at lower pressure. This stage is manifested by the appearance of cordierite after garnet, fibrolite (Sil2) after biotite in gneiss and transformation of Hbl1 into Opx2 and Cpx2 in granulite. Coronas of symplectitic Opx2 + Pl2 surrounding Grt1 and Cpx1 in mafic granulite are interpreted as products of near-isothermal decompression. The P–T–t path may be related tectonically to waning extension of the crust by the end of the early Proterozoic.

Granulite xenoliths from western Saudi Arabia: the lower crust of the late Precambrian Arbian-Nubian Shield

Abstract: Mafic and intermediate granulite xenoliths, collected from Cenozoic alkali basalts, provide samples of the lower crust in western Saudi Arabia. The xenoliths are metaigneous two-pyroxene and garnet granulites. Mineral and whole rock compositions are inconsistent with origin from Red Sea rift-related basalts, and are compatible with origin from island arc calc-alkaline and low-potassium tholeiitic basalts. Most of the samples are either cumulates from mafic magmas or are restites remaining after partial melting of intermediate rocks and extraction of a felsic liquid. Initial87Sr/86Sr ratios are less than 0.7032, except for two samples at 0.7049. The Sm-Nd data yield TDM model ages of 0.64 to 1.02 Ga, similar to typical Arabian-Nubian Shield upper continental crust. The isotopic data indicate that the granulites formed from mantle-derived magmas with little or no contamination by older continent crust. Calculated temperatures and pressures of last reequilibration of the xenoliths show that they are derived from the lower crust. Calculated depths of origin and calculated seismic velocities for the xenoliths are in excellent agreement with the crustal structure model of Gettings et al. (1986) based on geophysical data from western Saudi Arabia. Estimation of mean lower crustal composition, using the granulite xenoliths and the Gettings et al. (1986) crustal model, suggests a remarkably homogeneous mafic lower crust, and an andesite or basaltic andesite bulk composition for Pan-African juvenile continental crust.

Crustal trajectory of sapphirine-bearing granulites from Ganguvarpatti, South India: evidence for an isothermal decompression path

Abstract: Ganguvarpatti is part of a Precambrian terrane characterized by granulite facies rocks, including charnockites, mafic granulites, sapphirine-bearing granulites, leptynites and gneisses. A sequence of reactions deduced from the multiphase reaction textures provide information on the metamorphic history of this area, as they formed in response to decompression during uplift. Geothermobarometry and constraints from reaction textures define a segment of a P–T path traversed by the granulites of Ganguvarpatti. Near-peak metamorphic conditions of c. 800°C and 8 kbar were succeeded by a symplectitic stage at a significantly lower pressure (c. 700°C and 4.5 kbar), documenting a nearly isothermal decompression P–T path and rapid uplift (c. 12 km) followed by cooling. The presence of many fluid inclusions of extremely low density in the charnockites is consistent with a nearly isothermal uplift path. Attainment of a maximum pressure of c. 8 kbar indicates c. 27 km depth of burial during metamorphism. This would imply a total crustal thickness of c. 65–70 km at 2.6–2.5 Ga. Such a profound crustal thickness and a clockwise decompressive P–T path is interpreted as a consequence of tectonic thickening of crust, accomplished by collision tectonics of the southern granulite terrane against the Dharwar craton along the Palghat–Cauvery shear zone via northward subduction.